As FIG. 6 shows, the annular lamp E of the conventional auto license plate holder F is clamped by the opposed arcuate clamping pieces C, D of base frame A and cover frame B to take its position therein. But the problem is that annular lamp E is a glass tube flexed into shape which is very fragile. Under the differently tight or loose conditions of clamping pieces C, D, annular lamp E is certainly very susceptible to any shocking from transportation or use. A record is even found of 40% damaged lamps in shipment of an export container. Such sever loose makes improvement of lamp of the auto license plate holder greatly necessary.